''Friends'' may be winding down, but the show's cast is keeping busy planning their post-Must-See-TV lives:HOW YOU SPINOFF DOIN'?Matt LeBlanc turned up at the Los Angeles premiere of the documentary ''Mayor of Sunset Strip,'' and spoke to TV's ''Extra'' about progress on his ''Friends'' spinoff, the aptly titled ''Joey.'' ''We cast one character [already], and do a pilot in April,'' LeBlanc told the program. Apparently he means two characters are now cast -- unless the part of Joey is still up for grabs. THE PRODUCERS Jennifer Aniston will coproduce an upcoming Western based on the Robert Hansen novel ''The Assassination of Jesse James by the Coward Richard Ford.'' Of course, it will star Brad Pitt, Aniston's husband (and partner in the production company) as the outlaw Jesse James. SHIRT DIRT In other Aniston news, online fans of the influential Detroit rock band the MC5 are irritated with the actress, according to the Cincinnati Enquirer. Why? Turns out the ''vintage'' MC5 shirt she wears in the current ''Friends'' opening credits is a replica made by Levi's. Also, the fans assume that Aniston doesn't really know who the band is. Since tabloid reports suggested last week that Pitt bought Aniston a home recording studio, we suggest she cover ''Kick Out the Jame'' to silence the skeptics.